1. Field of the Invention
This invention relates to a telephoto objective, and more particularly to a telephotographic objective.
2. Description of the Prior Art
In designing a telephotographic lens, reduction in size and decrease in the weight of such lens continue to be design parameters. The increased secondary spectrum due to decreasing the telephotoratio has been successfully corrected, by using large fluorite crystals or an anomalous dispersion glass. However, in focusing, it is conventional to telescopically move the entire optical system. If the amount of this movement is designated by x, the distance of the system from an object to be photographed is designated by R, and the focal length is designated by f, the relationship EQU x= f.sup.2 /R
is satisfied. When R is constant, x increases as the square of f. In case of a super telephotographic objective for a 35 millimeter still-camera having a focal length larger than 300 millimeters, the movement of the objective will be larger, but the closest distance between the object and the camera is limited in view of the mechanical structure including the shift of the position of iris.
In order to overcome this deficiency, the movement of the front lens or a part of the lens system is utilized for focusing as in the case of a zoom lens system. For example, in case of a typical telephotographic lens consisting of a first convergent lens group and a second divergent lens group, the telephoto-ratio is made less than unity when f1&lt;F, where f1 is the focal length of the first convergent lens group and F is the total focal length of the whole lens system. Consequently, the amount of movement of only the first convergent lens group for focusing is less than that of the whole system.
It is usual to share the burden of correcting aberration evenly by the first and second groups. When only the first group is moved, however, the variation in aberration in response to a focusing operation becomes so significant that this approach is impractical. A realizable first lens group will have an extremely deviated aberration. It has been impossible to design the second lens group to correct the deviated aberration so as to obtain the balanced aberrations for the whole lens system. On the other hand, when the second lens group is intended to be moved for focusing, the height of an incident ray at the center light flux relative to the first and second groups varies in response to the amount of the movement of the second group so that it is necessary to closely correct the various aberrations including spherical aberration of both groups; and this is extremely difficult to realize with a limited number of lenses. These conventional types of systems may be realized in accordance with Gaussian optical theory, but no satisfactory solution to aberration problems has been found.